(a) Field of the Invention
The present invention relates to a polystyrene-base foamed composite sheet having heat-shrinkable properties.
(b) Prior Art of the Invention
Heat-shrinkable polystyrene-base foamed sheets have been widely used for covering labels of bottles and plastic vessels. When the sheets are employed for various uses, the sheets are often printed prior to use. Polystyrene-base foamed sheets are substantially opaque and hence printing must be conducted on the surface of the foamed sheets. As a result, the printed surface is rubbed when the printed labels are fitted onto the vessels or when charging contents into the label-covered vessels, which leads to problems such as scraping off of printed patterns or transfer of ink. There is a need in the art to resolve these problems.
On the other hand, it has been proposed that a composite sheet be prepared by laminating a polystyrene-base foamed sheet and a polystyrene-base film, and that printing be applied between the foamed sheet and the film. As a method of the above lamination, attempts have made to heat the polystyrene-base film and directly weld the same onto a polystyrene-base foamed sheet or to stick the polystyrene-base film on the polystyrene-base foamed sheet by using a thermoplastic adhesive resin as a binder.
In the above methods, however, heat is applied when a heat-shrinkable polystyrene-base foamed sheet and a heat-shrinkable polystyrene-base film are laminated, which leads to shrinkage of the foamed sheet and the film. Consequently, the composite sheet obtained by the above process causes deformation of the printed patterns or becomes unstable and deviates from the prescribed dimensions.
In addition, the foam cells in the boundary layer of the polystyrene base foamed sheet to be adhered expand in the operation of thermal adhesion. Thus, essential surface smoothness of the polystyrene-base film is impaired and the commodity value of the composite sheet is extremely decreased.
Furthermore, the composite sheet having impaired activity and enlarged irregularity on the surface causes much loss and trouble when the composite sheet is wound onto the vessels. For example, the vessels can be covered with the composite sheet at a rate of 100 vessels per minute by simultaneously cutting a continuous sheet into a prescribed length immediatly prior to winding the same onto vessels. In such a step, the front part of the composite sheet is placed under suction and the rear end of the sheet is cut with a cutter. A sheet having surface irregularity cannot be sufficiently sucked which leads to an irregular cut surface.
Additionally, in order to reduce the amount of heat required for adhesion, attempts have been made to apply thermoplastic resin base adhesive. However, labels composed of such composite sheet cause delamination at the interface of the foamed sheet and the film during subsequent heat treatment such as pasteurization. Consequently, restriction is imposed upon the use of such composite sheets.